In general, when a voltage of both terminals of an electric cell (a battery cell) exceeds a predetermined value, there is a risk of explosion, and when the voltage drops below the predetermined value, eternal damage occurs in the battery cell. Since a hybrid electric vehicle, a notebook computer and the like require relatively large electric power, when electric power is supplied using the battery cell, a battery module (a battery pack) obtained by serially connecting battery cells is used. However, when such a battery module is used, voltage unbalance may occur by performance deviation of the battery cells.
At the time of charge of the battery module, when one battery cell firstly reaches an upper limit voltage as compared with other battery cells in the battery module, since it is not possible to charge the battery module any more, it is necessary to end the charge in the state in which the other battery cells have not been sufficiently charged. In such a case, the charge capacity of the battery module does not reach rated charge capacity.
At the time of discharge of the battery module, when one battery cell firstly reaches a lower limit voltage as compared with other battery cells in the battery module, since it is not possible to use the battery module any more, the use time of the battery module is shortened.
As described above, at the time of charge or discharge of the battery module, electric energy of a battery cell having higher electric energy is supplied to a battery cell having lower electric energy, so that it is possible to improve the use time of the battery module, wherein such an operation is called battery cell balancing.
FIG. 1 is a diagram illustrating a battery cell balancing circuit using a parallel resistor according to the conventional art. As illustrated in FIG. 1, the battery cell balancing circuit includes a battery module 11 having battery cells CELL1 to CELL4 serially connected to one another, resistors R11 to R14 serially connected to one another, and switches SW11 to SW15 that selectively connect both end terminals of the battery module 11 and respective connection terminals among the battery cells CELL1 to CELL4 to respective corresponding terminals of the resistors R11 to R14.
Referring to FIG. 1, at the time of charge of the battery module 11, when a charged voltage of an arbitrary battery cell of the battery cells CELL1 to CELL4 in the battery module 11 firstly reaches an upper limit voltage as compared with charged voltages of the other battery cells, a corresponding switch of the switches SW11 to SW15 is turned on, so that the charged voltage is discharged through a corresponding resistor of the resistors R11 to R14.
For example, when a charged voltage of the second battery cell CELL2 firstly reaches an upper limit voltage as compared with charged voltages of the other battery cells CELL1, CELL3, and CELL4, the switch SW12 is turned on. Accordingly, the charged voltage of the second battery cell CELL2 is discharged through the resistor R12, so that battery cell balancing is achieved.
However, in the case of using such a battery cell balancing circuit, since electric power is consumed through a resistor, efficiency is reduced. Furthermore, since it is not possible to supply an upper limit voltage to a battery cell having a low voltage during the use of a battery module, efficiency is reduced.
FIG. 2 is a diagram illustrating a battery cell balancing circuit using a capacitor according to the conventional art. As illustrated in FIG. 2, the battery cell balancing circuit includes a battery module 21 having battery cells CELL1 to CELL4 serially connected to one another, capacitors C21 to C23 serially connected to one another, and switches SW21 to SW24 that selectively connect one side terminal of the capacitor C21, a connection terminal between the capacitors C21 and C22, a connection terminal between the capacitors C22 and C23, and the other side terminal of the capacitor C23 to one of both terminals of each of the battery cells CELL1 to CELL4.
Referring to FIG. 2, the battery cell balancing circuit using a capacitor has two connection states. In the first connection state, the one side terminal of the capacitor C21, the connection terminal between the capacitors C21 and C22, the connection terminal between the capacitors C22 and C23, and the other side terminal of the capacitor C23 are respectively connected to one side terminal (a positive terminal) of each of the battery cells CELL1 to CELL4 as illustrated in FIG. 2. In the second connection state, the one side terminal of the capacitor C21, the connection terminal between the capacitors C21 and C22, the connection terminal between the capacitors C22 and C23, and the other side terminal of the capacitor C23 are respectively connected to the other side terminal (a negative terminal) of each of the battery cells CELL1 to CELL4.
However, such a battery cell balancing circuit has a problem that efficiency is low because a hard switching operation is generated between the capacitors and the battery cells. It is preferable that capacities of the battery cells in the battery module are equal to one another, but the capacities of the battery cells become different from one another due to various factors. In such a case, even though a charged voltage of a certain battery cell is lower than a charged voltage of another battery cell, it may have a larger capacity. In such a case, it is necessary to transfer a voltage of a battery cell having a high voltage to a battery cell having a low voltage. However, in such a conventional battery cell balancing circuit, it is not possible to perform such a voltage transfer function.
FIG. 3 is a diagram illustrating a battery cell balancing circuit using a fly-back structure according to the conventional art. As illustrated in FIG. 3, the battery cell balancing circuit includes a battery module 31 having battery cells CELL1 to CELL4 serially connected to one another, a fly-back converter 32, switches SW31 to SW34 that selectively connect a plurality of secondary coils of the fly-back converter 32 to both terminals of each of the battery cells CELL1 to CELL4, and a switch SW35 that selectively connects both ends of a primary coil of the fly-back converter 32 to both ends of the battery module 31.
The battery cell balancing circuit of FIG. 3 is a battery cell balancing circuit using a fly-back structure belonging to SMPS (Switch Mode Power Supply) and has a structure in which it is possible to transfer electric energy to the battery cells CELL1 to CELL4 serially connected to one another in the battery module 31 by using the switches SW31 to SW34, and it is possible to transfer electric energy between both end terminals of the battery module 31.
Since such a battery cell balancing circuit has a SMPS type, it has superior efficiency. However, as the number of battery cells included in the battery module increases, the size of a magnetic core used in the fly-back converter becomes large. Therefore, the cost of the battery cell balancing circuit becomes expensive.